BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetostatic wave devices such as magnetostatic resonators, magnetostatic wave filters, etc.
2. Discussion of the Background
A magnetostatic wave device comprises a ferrimagnetic film formed of YIG or the like, a transducer that is an electrode for radiating electromagnetic waves to the ferrimagnetic film, and a transmission line for feeding high-frequency wave signals to the transducer. As microwave or quasi-microwave signals are supplied to the transducer, the resulting electromagnetic waves propagate in the ferrimagnetic film upon converted to magnetostatic waves. Since the frequency of the magnetostatic waves is dependent on the intensity of an external magnetic field applied to the ferri-magnetic film, the magnetostatic wave device may be allowed to function as a resonator or filter by control of the intensity of the applied magnetic field.
In JP-A""s 10-75107 and 11-67540, the inventors have come up with a magnetostatic wave device which can be reduced in size and excellent in function as well. FIGS. 1A and 1B are a perspective view of the magnetostatic wave device set forth in JP-A 11-67540 and a sectional view as taken along the line Bxe2x80x94B in FIG. 1A, respectively.
This magnetostatic wave device comprises a ferrimagnetic film 1 for exciting and propagating magnetostatic waves, an RF signal feed line 2 mounted on the surface of the ferrimagnetic film 1, and a magnetic field generator for applying a magnetic field to the ferrimagnetic film 1. The magnetic field generator comprises a permanent magnet 6 for applying a fixed magnetic field to the ferrimagnetic film 1, a coil 7 for applying a variable magnetic field thereto, and a pair of yokes 4a and 4b which are opposite to each other while an air gap with the ferrimagnetic film received therein is provided between them. One ends of the pair of yokes 4a and 4b are opposite to each other with the permanent magnet 6 sandwiched between them and the other ends are opposite to each other with non-magnetic, electrically conductive posts 91 and 92 sandwiched between them. That is, the pair of yokes 4a and 4b are opposite to each other while the air gap with the ferrimagnetic film 1 received therein is provided between them. The posts 91, 92 and the permanent magnet 6 work as supporting members. The yoke 4a is provided with protrusions 41a and 42a whereas the yoke 4b is provided with protrusions 41b and 42b. The opposite protrusions 41a and 41b form together a magnetic pole pair while an air gap 81 with the ferrimagnetic film 1 received therein is provided between them. A conductor film (not shown) is provided all over the surfaces of protrusions 41a and 41b and at least a surface portion of the yoke in the vicinity of protrusions 41a and 41b. The opposite protrusions 42a and 42b, around which a coil winding is wound to from a coil 7, form together a magnetic pole pair while an air gap 82 is provided between them. In this magnetostatic wave device, the length La of the air gap 81 is usually smaller than the height of the permanent magnet 6; a magnetic flux generated from the coil 7 passes primarily through a magnetic path that does not pass through the permanent magnet 6, viz., a magnetic path defined by air gap 82-yoke 4a (protrusion 42a-protrusion 41a)-air gap 81-yoke 4b (protrusion 41b-protrusion 42b), so that a fixed magnetic field due to the permanent magnet 6 and a variable magnetic field due to the coil 7 can be applied to the ferrimagnetic film 1. By control of the amount of currents passing through the coil 7, it is thus possible to change the resonance frequency of magnetostatic waves additively or subtractively from the frequency corresponding to the intensity of the fixed magnetic field. With this magnetostatic wave device, the magnetic resistance of the magnetic path through which the variable magnetic field passes can be reduced because the magnetic flux generated by the coil 7 does hardly pass through the permanent magnet 6. It is thus possible to reduce the number of turns forming the coil and, hence, reduce the overall size of the magnetic circuit. Further, the overall device can be more slimmed down as compared with a device with a permanent magnet in series with a coil. Furthermore, if a plurality of air gaps, each receiving a ferrimagnetic film therein, are disposed with varying lengths, magnetic fields having varying intensities can then be applied to the ferrimagnetic films in the respective air gaps, so that the respective ferrimagnetic films can be excited in resonance at varying frequencies. For instance, this arrangement may be applied to a VOC that oscillates at two or more discrete frequency bands.
Variations in the length La of the air gap 81 or the length Lb of the air gap 82 in the illustrated magnetostatic wave device cause a change in the intensity of the magnetic field in the air gap with the ferrimagnetic film 1 received therein, which may otherwise cause a change in the resonance frequency of magnetostatic waves. Even when external force is applied to this magnetostatic wave device, however, there is a little variation in the gap lengths La and Lb, because one pair of yokes 4a and 4b are supported by the posts 91 and 92.
In the magnetostatic wave device having the structure shown in FIGS. 1A and 1B, both end faces of the posts 91 and 92 are bonded to the major surfaces of the yokes 4a and 4b by means of an electrically conductive adhesive layer. However, it is difficult to form this conductive adhesive layer with uniform thickness. Thus, even when the height of each of the magnetic pole-forming protrusions from the major surface of the yoke is within tolerance, the air gap lengths La and Lb often go out of tolerance. Consequently, it is often difficult to fabricate a magnetostatic wave device having a given resonance frequency in high yields. Since this magnetostatic wave device is designed in such a way that one pair of yokes are supported by the magnet 6, the distance between both yokes is affected by the thickness of the magnet 6 as well. However, it is difficult to place the heights of the posts 91 and 92 and the thickness of the magnet 6 simultaneously within tolerance; the air gap lengths are likely to vary.
As the magnetostatic wave device decreases in size, for instance, with each of the yokes 4a and 4b having a major surface size of about 10 mm2 and a thickness of about 0.5 mm, the yokes are susceptible to deflection and warpage and, as a consequence, the air gap lengths are susceptible to variations.
In the conventional magnetostatic wave device as explained above, the posts 91 and 92 that connect one pair of yokes 4a and 4b together are each formed of a nonmagnetic, electrically conductive material for the reasons that any magnetic connection is prevented from being made between both yokes by way of the posts and both yokes are placed at the same potential to reduce high-frequency wave signal losses. However, when each post is all formed of a non-magnetic, electrically conductive metal such as brass or copper, there is an additional increase in the weight of the magnetostatic wave device having heavy parts such as yokes, a permanent magnet and a coil, which leads to another problem that users"" demands for weight reductions and slimming-down of electronic parts cannot be met. The electrical resistance of the posts should preferably be as low as possible. However, it is desired to develop means capable of enlarging the surface areas of the posts so as to reduce electrical resistance at high frequencies because high-frequency currents pass only in the vicinity of the surfaces of the posts; they do not reach to deep portions of the posts.
It is here understood that when the posts 91 and 92 are each formed of a lightweight yet non-conductive material such as a ceramic material and provided on its surface with a conductor film for imparting conductivity thereto, any weight increase may be reduced. In this case, too, the means capable of enlarging the surface areas of the posts are still desired.
A first object of the present invention is to reduce variations in the performance of a magnetostatic wave device which enables the size of a magnetic field generator to be reduced with no fluctuation in its performance. A second object of the invention is to achieve weight reductions with reduced signal losses while the first object of the invention is accomplished. A third object of the invention is to achieve reductions in the fabrication cost of the magnetostatic wave device while the first object is accomplished optionally with the second object of the invention. A fourth object of the invention is to provide a magnetostatic wave device which can be easily mounted on electric and electronic equipment while the first object is accomplished optionally with the second object of the invention.